1. Field of the Invention
The present invention generally relates to zero-voltage-transition (ZVT), 3-phase voltage link converters (VLC) and, more particularly, to 3-phase voltage link converters in which ZVT can be achieved without affecting the switching modulation scheme of the main power switches.
2. Description of the Prior Art
Three-phase PWM voltage link converters (VLC), including voltage source inverters (VSI), boost rectifiers and voltage source active power filters, are widely used in medium to high power conversion and power conditioning systems. FIG. 1 shows an example of a prior art three-phase VLC. To achieve high performance, low weight, and small size, a high switching frequency is preferred. Unfortunately, high frequency operation seeds severe problems, including, excessive switching losses, diode reverse recovery, and high dV/dt in the load. These problems reduce circuit efficiency, produce high EMI emission, and can cause premature failure of isolation material in electric motors connected to the VLC. To avoid the problems associated with high frequency switching, the switching frequency of most conventional VLC's is usually held below 10 KHz in most high power applications.
It is recognized that high switching frequency losses stem mainly from an abrupt voltage change experienced in the VLC when the active switch turns on. In recent years soft switching techniques, such as zero voltage switching (ZVS), have been developed to reduce switching losses and allow higher switching frequencies to be used. There are many advantages to using ZVS in VLCs. ZVS practically eliminates diode reverse recovery, switch turn-on losses, and allows the use of larger capacitor snubber to reduce switch turn-off losses and dV/dt in output voltage.
Much research effort has been spent on implementing soft switching techniques of VLC in recent years and several ZVS topologies have been proposed. Since most ZVS problems stem from the "stiff" DC bus voltage, most of the ZVS topologies try to "soften" the voltage source by inserting an interface circuit between voltage source and bridge switches. The interface circuit resonates the DC bus voltage to zero before switch turn-on to obtain zero voltage switching. This is the basic concept of DC Link Commutation, used in, for example, resonant DC link inverters (RDCLIs). Although switching losses can be reduced in RDCLIs due to zero voltage switching, other problems have been created such as, for example, high circulating energy, high switch voltage stress, and poor PWM control resolution.
ZVS circuit topologies are known which possess full range PWM control capability. However, the soft switching of inverter switches in these topologies is achieved by an abrupt turn-off of an auxiliary switch on the DC bus. Since this DC bus switch is in the main power path, considerable conduction loss and turn-off loss are introduced. Also, the synchronization of switch turn-on in these topologies results in non-optimum PWM scheme. To remedy these problems a ZVS rectifier with a DC rail diode, which helps to realize DC link soft switching has been introduced in the art. The DC rail diode scheme is very effective but has the disadvantage that it renders the ZVS rectifier only to provide unidirectional power conversion.
A common disadvantage of DC link commutation converters is that some auxiliary device has to be put in the main power path. This can be overcome by putting the soft switching circuit on the AC side of VLC. The normal operation of the AC-side-commutation converters is quite similar to that of their PWM counterparts. The soft switching circuit is active only at the short commutation transient from diodes to switches. This concept is the same as Zero-Voltage Transition (ZVT) technique in DC--DC converters.
Two ZVT topologies include an auxiliary resonant commutated inverter (ARCPI) and a 3-phase ZVT PWM rectifier/inverter as shown in FIG. 2 and FIG. 3, respectively. A major concern is how to simplify the auxiliary circuit 10, and to avoid the extra turn-off loss in the previous three-phase ZVT PWM rectifier/inverter.
In the case of the ARCPI, shown in FIG. 2, the concept of AC side commutation is utilized. This inverter can be controlled according to any PWM scheme with some special switching arrangement for ZVT transition. The operation of soft switching is independent in every phase. Suppose D.sub.1 (the parallel diode of S.sub.1) is conducting and S.sub.2 should be turned on. The ZVT commutation proceeds as follows:
a) At the beginning, S.sub.1 and S.sub.xa are turned on. Then resonant inductor L.sub.xa will be charged through D.sub.1 ;
b) When the inductor current reaches load current of Phase A, D.sub.2 is turned off naturally;
c) When the inductor current equals load current plus a "trip" current, S.sub.1 is turned off, and the resonance between L.sub.a and the capacitance of node A begins;
d) When the node voltage V(A) resonates to negative bus voltage, D.sub.2 starts conducting and S.sub.2 can be turned on under zero voltage. After this, L.sub.xa is discharged and its current decreases towards zero;
e) S.sub.xa is turned off when its current reaches zero; then the ZVS commutation is completed.
ARCPIs have the advantage that auxiliary switches 14 (S.sub.xa, S.sub.xb, and S.sub.xc) need only withstand half of the DC bus voltage and are turned off under zero current condition. However, six auxiliary switches are required and usually not cost effective except in very high power applications.
The ZVT converter shown in FIG. 3 attempts to overcome the drawback of the ARCPIs shown in FIG. 2. The ZVT converter shown in FIG. 3 uses only one auxiliary switch S.sub.x. However, there are some tradeoffs. The most serious being that the ZVT converter cannot operated when three high side switches or three low side switches in the power stage 12 are conducting (i.e. in a zero voltage vector), because no voltage is applied at the ZVT circuit input. To overcome this limitation, a space vector modulation (SVM) scheme has been modified so that the commutation from diodes D.sub.1 -D.sub.6 into active switches occurs in all three phases at the same time. For example, if the current in phase A is flowing into the bridge and phase B and phase C currents are flowing out of bridge, then the only turn-on commutation is from D.sub.1, D.sub.4, D.sub.6 to S.sub.2, S.sub.3, S.sub.5. The commutation process is divided into the following steps:
a) At the beginning of the commutation, S.sub.x, S.sub.1, S.sub.4, and S.sub.6 are turned on, so resonant inductor are charged by V.sub.i ;
b) Just as in ARCPI, when resonant inductor currents (L.sub.a, L.sub.b, and L.sub.c) reach certain values, S.sub.1, S.sub.4 and S.sub.6 are turned off, so the resonant inductor resonate with the node capacitances (A, B, and C);
c) After all three node voltages (A, B, and C) resonate to opposite rails, S.sub.2, S.sub.3 and S.sub.5 can be turned on under zero voltage condition. The resonant inductor (L.sub.a, L.sub.b, and L.sub.c) are discharged by V.sub.i in this operation mode;
d) Before any resonant inductor current decreases to zero, the lone switch S.sub.x is turned off. The remaining currents in the resonant inductor continue to be discharged through feedback diodes 16 to zero;
e) After the three resonant inductor currents reach zero, the ZVT commutation is completed, and the converter resumes its normal operation as a conventional PWM converter.
A serious drawback of the aforementioned topologies is that more switching action of the main switches is required to ensure the ZVS condition. As a result, more turn-off losses and control complexity are introduced, thereby limiting efficiency.